1. Field of the Invention
This invention relates to a micellar slug suitable for use in a micellar drive for recovering oil from subterranean reservoirs. More specifically, it relates to a micellar slug capable of forming micro-emulsions at a high salt concentration, of largely decreasing the interfacial tension thereof, and of improving the oil recovery efficiency.
2. Description of the Prior Art
It is wellknown in the art that the so-called "primary recovery" methods, including pumping methods, can recover only a portion of petroleum or crude oil (which is referred as "oil" herein) from subterranean reservoirs and leave substantial amounts of oil in the subterranean reservoirs.
In order to recover the remaining large amounts of oil from the subterranean reservoirs, the so-called "secondary recovery" methods have been proposed. For example, water or gas is injected into subterranean reservoirs from an injection well at a pressure sufficient to increase the flowability of the oil, steam is injected into subterranean reservoirs so as to effect the displacement of oil toward a production well, or oil in subterranean reservoirs is partially burned to heat the subterranean reservoirs so as to decrease the viscosity of the oil and increase the flowability of the oil. Thus, by means of these methods, oil is recovered from subterranean reservoirs. Furthermore, the so-called various "tertiary recovery" methods, including a combination of secondary recovery methods and improved secondary recovery methods, utilizing surfactants or water-soluble polymers have also been proposed. These methods are generally called "enhanced oil recovery" (EOR) methods.
Of these EOR methods, the recent "micellar drive" methods are to be noted. According to these methods, a micellar slug, that is, clear micro-emulsion derived from water and oil such as petroleum, petroleum distillates or fuel oil, is injected under pressure into subterranean reservoirs for the recovery of oil in the subterranean reservoirs. These EOR methods are disclosed in, for example, U.S. Pat. Nos. 3,506,070, 3,613,786, 3,740,343, 3,983,940, 3,990,515, 4,017,405, and 4,059,154. These prior arts disclose that various kinds of surfactants including anionic-, nonionic-, and cationic-type surfactants can be used alone or in any mixture thereof in the formation of micellar slugs. Examples of such surfactants are petroleum sulfonates, alkylaryl sulfonates, dialkyl sulfosuccinates, alkane sulfonates, polyoxyethylene alkylether sulfates, alpha-olefin sulfonates, polyoxyethylene alkylethers, polyoxyethylene alkylphenylethers, polyol fatty acid esters, alkyltrimethyl ammonium salts, and dialkyldimethyl ammonium salts.
These surfactants used in the formation of micellar slugs must be available at a low cost since a large amount of micellar slugs is used for the recovery of oil from subterranean reservoirs. Furthermore, the surfactants should provide micellar slugs having small interfacial tensions both between slug and water and between slug and oil, and micellar slugs must have a viscosity equal to or slightly greater than that of oil, in order to transfer the oil remaining in the subterranean reservoirs to production wells. Especially taking into account the fact that an oil recovery operation becomes easier when the viscosity of the injected micellar slug is decreased, the micellar slug desirably has a viscosity similar to that of the oil remaining in the subterranean reservoirs.